Grades 9–12 · Earth & Space
Why does the Sun keep shining?
The sunlight arriving at Earth began with nuclear reactions deep inside a star held together by gravity. The Sun is hot plasma, not fuel burning in oxygen. Gravity compresses its interior, while pressure in the hot material supports it against further collapse. The core's high temperature and density allow hydrogen nuclei to undergo fusion, aided by quantum tunneling through their electrical repulsion.
In the diagramFusion powers the Sun from deep insideCoreEnergy moves outwardRadiation + convectionLight leaves photosphereNet nuclear change+4 protons → helium-4+ 2 positrons + 2 neutrinos1 net helium production26.7 MeV total releaseSome energy leaves in neutrinos26.7 MeV per chain includes positron annihilation.
The main reaction route is the proton–proton chain. Several steps together consume four protons and produce one helium-4 nucleus, two positrons and two electron neutrinos. Four protons do not have to collide at once. The positrons annihilate with electrons in the surrounding plasma, contributing to the overall energy account.
Including that annihilation, about 26.7 million electronvolts are released per net helium-4 nucleus produced. The complete rest-mass account decreases and the corresponding energy appears according to E = mc². Some of the released energy leaves with neutrinos, so the entire 26.7 MeV cannot be counted as heat deposited in the Sun. A per-reaction energy also does not give power without knowing how many reactions occur per second.
Energy retained in the star moves outward through repeated interactions between radiation and matter. Farther out, moving plasma carries energy by convection. The photosphere ultimately emits the light that reaches us. Gravity, pressure and continuing energy production therefore belong to one connected account: fusion helps replenish energy that the star keeps losing into space while its enormous mass remains gravitationally confined.
The illustration follows production and transport rather than timing the whole journey. The Sun's sustained light comes from a vast population of reactions, not one combustion flame or one photon traveling straight from core to Earth. The supplied net reaction and energy explain the source without calculating the Sun's total power or a transport time.
About this explanation
Starting knowledge: Know atomic nuclei, protons, neutrons and the difference between nuclear and chemical reactions.
Sources
- NASA Marshall Space Flight Center — The Solar Interior
- NASA — Nuclear fusion energy in the proton–proton cycle
- NASA — Origin and Nature of the Sun
- OpenStax — College Physics: Fusion